why doesn't RAID performance scale?


R

rmeden

Maybe some of the folks here can answer this...

Why doesn't RAID performance scale?

If a 7200RPM disk can write 20MB/s writes, why can't a stripe of 5
give 100MB/s?

If I use a controller with HW parity, surely it should be able to keep
up with a 6 disk RAID 5 volume and perform similar to a non-parity
stripe.

This is not just a single vendor...

I get about 60MB/s on a Sun 3511 array (big bucks)
I get around 45MB/s on a HP P400 (w/battery cache)
I've noticed similar problems on a Dell controller (don't remember the
model#)

In all cases, the controller write cache is enabled of course.

Shouldn't RAID scale?

Robert
 
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A

Arno Wagner

Previously said:
Maybe some of the folks here can answer this...
Why doesn't RAID performance scale?

It does.
If a 7200RPM disk can write 20MB/s writes, why can't a stripe of 5
give 100MB/s?

If it is a RAID1 stripe and the rest of the system can handle 100MB/s
wor mixed device access, you will get that performance.
If I use a controller with HW parity, surely it should be able to keep
up with a 6 disk RAID 5 volume and perform similar to a non-parity
stripe.

No. Some hardware controllers are surprisingly slow.
This is not just a single vendor...
I get about 60MB/s on a Sun 3511 array (big bucks)
I get around 45MB/s on a HP P400 (w/battery cache)
I've noticed similar problems on a Dell controller (don't remember the
model#)

First, not only the controller is a bottleneck. Second
is this reading or writing? And what is the performance of the
individual disks, when connected to this contoller?
In all cases, the controller write cache is enabled of course.
Shouldn't RAID scale?

As I said, it does. But hardware has kimuts. It does not scale
past these limits.

Here is a speed sample from a 8-way Arcea RAID6:

Read speed 340MB/s. That is 56.6MB/s for the individual disks.
I believe the disks are in the 60MB/s class. Looks like good
scaling to me. Maybe your hardware is PCI or is just over-priced?

Arno
 
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S

Stretch

Maybe some of the folks here can answer this...
Why doesn't RAID performance scale?

It doesn't?
If a 7200RPM disk can write 20MB/s writes, why can't a stripe of 5
give 100MB/s?

It can't?
If I use a controller with HW parity,
surely it should be able to keep up

Whatever 'it' is supposed to mean.
with a 6 disk RAID 5 volume

And why is that?
and perform similar to a non-parity stripe.

You mean, to RAID0? Didn't you just say that 'RAID' doesn't scale?

RAID5 writes of incomplete strip(e)s (or whatever a full complement of
blocks is called here) also require a read in order to calculate the parity.
This should not showup though in static benchmarks.
This is not just a single vendor...

I get about 60MB/s on a Sun 3511 array (big bucks)
I get around 45MB/s on a HP P400 (w/battery cache)
I've noticed similar problems on a Dell controller (don't remember the
model#)
In all cases, the controller write cache is enabled
of course.

Of course.
And now you are surprized that you are measuring the cache performance?
Shouldn't RAID scale?

Doesn't it?
 

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